Airplane engines are typically mounted below an aircraft wing or near the tail section by an engine mount. Mounts are usually provided for both the forward portion of the engine and the aft portion of the engine, so as to distribute the engine load. Typical engine mounts include several components. One of the components is a generally planar upper fitting that has a mounting platform located along the upper edge that is used to attach the engine mount to a support structure of the aircraft, e.g., a wing strut or tail pylon. Multiple clevises are located on the lower edge of the upper firing and also along a portion of an engine casing. Multiple links, pinned in the clevises of both the upper fitting and the engine casing, connect the engine to the support structure. Similar engine mounts of this type are used at both the forward and aft portions of the engine.
Engine mounts are designed to handle a variety of loads, during all phases of flight. The loads include vertical loads (the weight of the engine plus maneuver loads), axial loads (caused by the engine's thrust), side loads (caused by wind buffeting, for example), and torsion loads (caused by the rotary operation of the engine, or by the loss of a turbine blade). An engine mount must also accommodate thermal expansion and contraction of the engine relative to the mount. The effect of thermal expansion and contraction is most significant during cruise phase. During cruise, thermal expansion and contraction can cause an appreciable shift in the direction of forces acting on an engine mount.
Almost all airplane engine mounts are designed to be fail-safe, i.e., to prevent the engine from separating from the airplane. Fail-safe operation is provided by a secondary, or backup, load-carrying system. Two types of secondary systems are common. The first type utilizes components of the thrust reverser (such as the translating cowl) to carry engine loads. The second type utilizes catcher links placed within the engine mount itself. Catcher links are additional links in the engine mount that are typically unloaded during normal operation. Should a primary (i.e., non-catcher) link fail, the catcher links are capable of cooperating with the remaining unfailed links to carry engine loads. Link failures may result from many causes, including failure of pins or clevises; broken, deformed, missing, or mis-installed links; sheared pins; etc.
Between the two types of secondary systems, the thrust reverser system is the more widely used approach. Unfortunately, thrust reverser systems are structurally inefficient for the newest generation of large fan engines, such as those used on the Boeing 777. Because these engines can weigh more than 20,000 pounds, they require an additional 200 pounds of thrust reverser support structure in order to secure an engine if an engine mount fails. On airplanes, where weight and space are a premium, the use of catcher links is a more efficient solution, because they require relatively much less weight and space. Currently, relatively few catcher link engine mounts are known, and of these, only one is described as useful with extremely heavy engines. This is described in U.S. Pat. No. 5,275,357 (hereinafter referred to as the "'357 patent").
The '357 patent describes a three-link system, where the center link is the catcher link. The center link carries no load during normal operations, due to an oversized hole where the center link is attached to the engine casing. U.S. Pat. No. 5,303,880 (hereinafter "'880") is similar to the '357 patent, but with the addition of replaceable bushings. Although the systems disclosed in these patents appear capable of handling heavy airplane engines, the present invention has better horizontal loading capability. This capability allows the present invention to perform better during certain types of failures, such as a thrown-blade engine failure. As will be additionally appreciated from the following description, it is easier to install a vibration isolator in embodiments of the present invention than in embodiments of the '357 and '880 patents, should the need for such a device arise.
Another engine mount is known that includes two separate catcher links with translating spherical bearings. The design includes a total of five links: two vertical side links, one center horizontal link, and two smaller vertical catcher links sandwiched between each side link and the center link. However, to use this known two-catcher-link design on heavy engines, the bifurcation flow must be disadvantageously reduced in order to fit all five links between the bifurcation duct walls. In addition, during operation, if the center link fails, the two smaller vertical catcher links do not adequately carry the horizontal loads previously carried by the center link. This causes the catcher links to tend to "nut-crack", or shear, in their clevises. In addition, the catcher links require deep, narrow pockets in the upper fitting, which are difficult to machine. The catcher link spherical bearing track must be custom machined. The spherical bearings require frequent maintenance because the catcher links are free to vibrate in their attachments to the mount.
Thus, there exists a need for a superior fail-safe engine mount capable of handling extremely heavy airplane engines, while at the same time having a width that does not significantly reduce bifurcation flow. The mount should provide adequate vertical and horizontal load bearing capability in the event of a link failure, should be lightweight, and should allow installation of a vibration isolator if needed. The mount should also require less maintenance than current mounts and less custom manufacturing. As will be appreciated by the following description, the present invention is directed to providing such a superior fail-safe engine mount.